1. Field of the Invention
The present invention relates to a liquid crystal active-matrix display device which has thin film transistors (hereinafter referred to as TFTs) as addressing devices for controlling the transmittance of picture elements. More particularly, it relates to a liquid crystal active-matrix display device which has, as addressing devices, reverse stagger type TFTs whose semiconducting film is made of amorphous silicon (a-Si).
2. Description of the Prior Art
FIGS. 3a and 3b show a conventional liquid crystal active-matrix display device in which reverse stagger type TFTs are used as addressing devices. This liquid crystal display device comprises a pair of cell substrates 120 facing each other and a liquid crystal layer 103 sealed in the gap between the pair of cell substrates 120. One of the two cell substrates 120 comprises an insulating substrate 101, gate electrodes 102, a gate insulating film 105, an a-Si (amorphous silicon) semiconducting film 106, an insulting film 107, an n.sup.+ -a-Si (n.sup.+ -amorphous silicon) contact film 108 for ohmic contact, source and drain electrodes 109, picture element electrodes 110 for display, and a protective film 111. The picture element electrodes 110 are arranged in a matrix format. An additional capacity electrode Cs parallel to the liquid crystal capacity is formed to improve picture element potential-retaining characteristics and to minimize the shift in the picture element electrode potential level at the time of the fall of the gate driving pulse resulting from the capacity between the gate electrode 102 and the drain electrode 109 (Proc. Japan Display '83, p.4l2 (1983) and Proc. Euro Display '84, p.145 (1984)). The additional capacity electrode Cs is obtained by forming transparent conducting film in two layers on the insulating substrate 101 with the extension of the gate insulating film 105 between the two layers for isolation from each other. The first layer of the conducting film server as a ground electrode 112 and the second layer serves as the picture element electrode 110, as shown in FIG. 3b.
In the above-mentioned conventional liquid crystal display device, as mentioned above, the ground electrode 112 for providing the additional capacity Cs is made of transparent conducting film, so that the resistance of the ground electrode 112 for additional capacity Cs becomes high. This causes insufficient performance or a cross talk among the picture electrodes. The resistance of the ground electrode 112 for additional capacity Cs could be reduced by increasing the thickness or width of the ground electrode 112. However, the greater film thickness would result in a greater difference in level between the ground electrode 112 and the insulting substrate 101, which results in an increase in defects of step coverage of the gate insulating film 105 and other films on the ground electrode 112. The defects of step coverage would increase a short circuit between the ground electrode 112 and the picture element electrode 110 and increase the chance of breakage of the source electrode and other elements of each TFT to be formed in the later process. The greater electrode width would result in the higher probability of a short circuit between the ground electrode 112 for additional capacity Cs and the picture element electrode 110, and between the ground electrode 112 and the source electrode 109. It would also result in a larger capacity between the ground electrode 112 for additional capacity Cs and the source electrode 109 thus causing the cross talk in signals and a large load on the source driver.
In addition, the thicker or wider ground electrode 112 for additional capacity Cs, would involve a larger number of manufacturing processes, which is not favorable.